It has long been known to use electric furnaces for melting scrap or other ferrous raw material and reducing the resulting molten-metal bath, if appropriate with the addition of alloying elements, until a metal of specific composition is obtained.
In general terms, an electric furnace comprises a vessel limited by a sidewall and a bottom, covered with a hearth made of refractory material, and closed by a removable, vaulted cover, through which passes at least one electrode which is usually consumable and which consists of a graphite bar mounted so as to be vertically slidable, so that it can descend within a furnace batch, normally scrap, which is in contact with at least one fixed electrode located in the hearth.
In the case of a single-phase alternating-current furnace or a continuous-current furnace, the consumable electrode and the hearth electrode are connected to the two poles of a current source.
In the case of a two-phase or three-phase alternating current furnace, the consumable electrodes are connected to the poles of the current source, and the batch in contact with the hearth electrode constitutes the neutral conductor of the system.
One or more electric arcs thus form between the batch and each consumable electrode, and these cause the melting of the scrap and the formation of a molten-metal bath in the bottom of the vessel.
Until now, furnaces supplied with alternating current have been preferred, but it has been found that feeding the electrodes with direct current afforded many advantages, such as a reduction in noise and an increase in energy efficiency, because it is possible to use voltages higher than those allowed with alternating current.
However, to date, the use of very high direct current intensities has been avoided, because, since the currents always circulate in the same direction in the conductors, the electrodes and the bath generate considerable magnetic fields which deflect the arcs. Furthermore, for high powers several hearth-electrodes and return conductors, usually three, are used, and these likewise generate fields which exert substantial deflection effects on the arcs.
As long as the batch is in the form of scrap, the electrodes penetrate into the latter, at the same time digging in it pits which, in a sense, insulate the arcs from one another and promote their stability. By contrast, when the batch is melted completely, the arcs subjected to the magnetic effects generated as a result of the passage of current through the electrodes, the conductors and other parts of the apparatus can form in upredictable directions and are therefore highly unstable.
The zone in which the arcs form and which is at the highest temperature consequently cannot be kept in the center of the furnace, the walls and bottom of which can be subjected to excessive temperatures and to considerable wear of the refractory.
To overcome these disadvantages, the aim until now has been to provide installations which are as symmetrical as possible, so that the magnetic fields generated as a result of the circulation of the current in the various conductors balance one another and the arc or arcs are kept vertical.
For example, in DE-A-3,414,392, all the conductors are made to arrive at one and the same point which is located underneath the bottom of the vessel on the axis of the latter, and from whence the conductors radiate in symmetrical directions, the negative conductors rising vertically along the sidewall and then linking up with flexible conductors for connection to the consumable electrode located in the center of the vault.
Such an arrangement increases the length of the conductors and consequently the cost of the installation, and in practice it is very difficult to achieve perfect symmetry, since it is necessary to take into account not only the circuit of the conductors, but also many other disruptive influences.
Furthermore, this increases the crowding of the space which is located underneath the vessel and which it is preferable to keep free.